The Office of Foreign Assets Control (OFAC)

The Office of Foreign Assets Control (OFAC) is a financial intelligence and enforcement agency of the U.S. government charged with planning and execution of economic and trade sanctions in support of U.S. national security and foreign policy objectives. Acting under Presidential national emergency powers, OFAC carries out its activities against foreign states - such as Russia and Iran - as well as a variety of problematic organizations and individuals deemed to be a threat to U.S. national security.
Involvement of the U.S. Department of the Treasury in economic sanctions against foreign states dates to the War of 1812, when Secretary Albert Gallatin administered sanctions against Great Britain in retaliation for the harassment of American sailors.
The Division of Foreign Assets Control was established in the Office of International Finance by a Treasury Department order in 1950, following the entry of the People's Republic of China into the Korean War; President Harry S Truman declared a national emergency and blocked all Chinese and North Korean assets subject to U.S. jurisdiction. In addition to blocking Chinese and North Korean assets.
OFAC is responsible for administering the Specially Designated Nationals (SDN) List. The list is a publication of OFAC which lists individuals and organizations with whom United States citizens and permanent residents are prohibited from doing business.


ref:
http://en.wikipedia.org/wiki/Office_of_Foreign_Assets_Control